The present invention relates to a public telephone set for automatically transmitting alarm information corresponding to detection of an abnormality or fault when the abnormality is detected as a result of a self-diagnosis.
A self-diagnosis of a public telephone set (to be referred to as a telephone set hereinafter) is described in European Patent Publication No. 0012102 in detail. An alarm information transmission system in a telephone set is described in Japanese Patent Laid-Open No. 58-156259 in detail. In these prior art patents, an operation is started in response to formation of a DC loop upon an off-hook operation of a user.
Faults cannot be early found in telephone sets installed in backcountries and having low utilization efficiency since the telephone sets are not frequently used. When a message "out of order" is displayed, the telephone sets are not hooked off. The faults cannot be detected by a service or maintenance center unless the detects are acknowledged to the maintenance center through other ways. The "out-of-order" telephones are left unrepaired, and therefore, better service cannot be provided to the users.
In order to solve the above problem, a conventional telephone set is proposed in U.S. Pat. No. 4,644,109. In this telephone set, a ringing signal on the basis of ringing from the maintenance center is detected, a DC loop is automatically formed upon detection of the ringing signal, and a self-diagnosis can be automatically performed. When an abnormality is detected by this self-diagnosis, alarm information corresponding to the content of the abnormality and an assigned inherent address number are sent to an office line in the form of a predetermined signal.
In these conventional telephone sets, however, all incoming calls are regarded as polling from the maintenance center. Alarm information is transmitted in synchronism with the self-diagnosis upon reception of an incoming call. When the transmission operation is completed, the office line is disengaged from the telephone set. Therefore, communication in the disengaged state is allowed, and even if a call is not sent from the maintenance center but from a third party who wishes incoming communication, incoming communication cannot be undesirably performed.